Suspensions Flashcards
What is a suspension?
suspensions are dispersions in which insoluble drug particles (solid) are suspended in liquid vehicle (external phase)
disperse phase - insoluble solids
continuous phase - liquid medium
Why are suspensions used?
used for insoluble or poorly soluble drug
- volume of liquid required to dissolve the drug is too large
used due to the instability of some drugs in water
- preparation immediately before use
- limited shelf-life
What are the advantages of suspensions?
mask the taste of therapeutic agents
patients who have difficulty swallowing solid dosage forms
provide controlled drug delivery (intramuscular injections)
What are the characterises of a stable suspension?
good wettability
- depends on the contact angle
- required wetting agents
sedimentation velocity
- stoke’s law
- particle size = smaller is better
- viscosity of continuous phase (vehicle) = higher is better
sediment re-dispersibility
- depends on flocculated and deflocculated systems
- require flocculating agents
How does powder wettability affect suspension stability?
determined by measuring the contact angle = between the water droplets and the powder
smaller contact angle is required = hydrophilic
- good spreadability
- easy dispersing
interfacial tension
- between liquid and solid
- must be reduced as it pulls sphere inward creating a more spherical shape
- require a less spherical shape
surface tension
- liquid and gas= must be reduced to create a less spherical shape
- solid and gas = not affected, pushes surface outwards
What are examples of wetting agents?
hydrophilic polymers
- cellulose
- acacia
increases viscosity of the continuous phase
co-solvents can be used to enhance solid wettabilty
How can rate of sedimentation be controlled? Why must it be controlled?
stoke’s law
- reduce particle size = less irritation, ease of administration, no coalesce
- increase viscosity of the continuous phase (vehicle) = prevents/slows movement of disperse phase, hydrophilic polymers, polysaccharides
- reduce density difference
must remain stable to get consistent uniform doses
- prevent overdose and underdose
How can sediment re-dispersibilty affect stability of suspension? What system is required?
aggregated clusters that form as the suspension sits must be re-dispersed to get a uniform dosage
deflocculated
- low sedimentation
- low volume
- sediment contains few voids
- sediment is difficult to redisperse
flocculated
- high sedimentation
- high volume
- sediment contains many voids
- sediment can be re-dispersed
- no caking
What are the possible interactions for control flocculation?
no interaction = slow individual sedimentation
repulsion > attraction = slow individual sedimentation, compact sediment
attraction > repulsion = irreversible interaction, coagulation
balanced attractive-repulsive forces - loose interaction, loos floccules/aggregates produced , re-dispersible sediment
What are flocculating agents?
electrolytes
surfactants
polymers
- starch
